How I Met Your Mother has had their fair share of big names on the show (Britney Spears, Jennifer Lopez, John Lithgow…to name a few). And tonight they're adding Katie Holmes to the list when she guest stars as the legendary Slutty Pumpkin from Ted's Halloween past.

So did the big name bring big attitude to the set? Neil Patrick Harris gives us the behind-the-scenes scoop…

She may be married to one of the biggest Hollywood stars ever, but there wasn't a trace of any ego on set when Mrs. Cruise came to HIMYM.

"She was great," NPH gushes to us. "She's a really smart actor, you know? She didn't just show up to do her lines and split. She came the week before and watched how the rehearsal day goes so that she would be more comfortable. She was actually doing it; she was totally committed to it. All her stuff is mainly with Josh (Radnor), and they have really strong chemistry together."

Enough chemistry to be the mother? These sneak peeks tells us no, but it'll be fun to put a face to the infamous Slutty Pumpkin. Maybe a different famous lady will come in to be the wife of unlucky-in-love Ted Mosby. "We're good with the hot starlets. We've had J.Lo and Katie Holmes and Katy Perry…we're the go-to for hot actress chicks. And they're all dating Ted!"

But HIMYM fans know that Barney shouldn't be bothered with whom Ted is dating. Because right now Robin (Cobie Smulders), the love of his life (we all know it's true!) is dating Neil's Harold and Kumar costar, Kal Penn. "He's got an eight-episode arc, and I don't know how I feel about it," Neil admits to us. "He's dating Robin on the show and I'm kind of into her on the show, so I'm not sure how I feel about this whole Kevin making out with Robin arc."

We know it tears Barney apart. We know it in our hearts and we won't accept this Nora business any longer! Sorry, we have a lot of pent-up emotion about this. When we brought up the possibility that his character marries Robin in the end and they live happily-ever-after, Neil gave us a very Barney answer. "Why would I want to marry her? Then I couldn't bang other chicks."